The aryl hydrocarbon receptor (AHR) binds a variety of pollutants, including 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD) and benzo(a) pyrene, and mediates the carcinogenic and toxic effects of these compounds. After binding ligand, AHR dimerizes with the aryl hydrocarbon receptor nuclear translocator (ARNT) protein. The AHR/ARNT dimer activates transcription of CYP1A1 and certain other genes. Induction of CYP1A1 is involved in carcinogenesis by benzo(a) pyrene. In contrast, the mechanism of TCDD carcinogenesis and toxicity is obscure. However, it probably depends (at least in part) upon transcriptional activation of certain other (unidentified) genes. In this application three specific aims are proposed. 1) Preliminary evidences indicates that the transcriptional co-activators steroid receptor co- activators 1,2 and 3 (SRC-1, -2 and -3) and thyroid receptor/retinoblastoma protein interacting protein (Trip230) may act as co-activators for the AHR/ARNT dimer. This possibility will be investigated further, for examined by ascertaining whether the endogenous co-activators bind endogenous ARNT and/or AHR in mammalian cells, and whether negation of function of each co-activator compromises transcriptional activation by AHR/ARNT. In addition, three novel ARNT-interacting proteins will be analyzed further, and novel co- activator for co-repressor proteins for AHR and ARNT will be sought. These studies should provide important insights into the mechanism of transcriptional activation by the AHR/ARNT dimer, may reveal cross-talk between AHR/ARNT and other transcription factors, and may explain some of TCDD's toxic actions. 2) We previously isolated mutants of the Hepa-1 mouse cell line. Class "B" mutants are probably defective in a transcription factor for AHR or a chromatin remodeling factor. The B gene will be cloned and characterized. These studies should provide important insight into the regulation of expression of the AHR gene. 3) Six novel TCDD-inducible genes have been isolated from Hepa-1 cells. Additional TCDD-regulatable genes will be isolated from mouse liver, mouse thymocytes, and human breast cancer cells. Using model cellular systems of TCDD toxicity, experiments will probe whether the products of these genes mediated TCDD toxicity.